Communication pubs.acs.org/crystal
Mn8@Na8 Cube-in-Cube SBBs-Based Heterometallic Coordination Network with Unprecedented (39.46)8 Topological Mn8(μ4‑OMe)6 Cubes Min-Min Liu, Cai-Yun Han, Ying-Lian Qin, and Xian-Ming Zhang* School of Chemistry & Material Science, Key Laboratory of Magnetic Molecules and Magnetic Information Material, Ministry of Education, Shanxi Normal University, Linfen 041004, P. R. China S Supporting Information *
ABSTRACT: A novel body-centered cubic antiferromagnetic heterometal-coordination network Na4Mn8(μ4-OMe)6(μ4-MeCO2)12 ·2MeCO2 based on Mn8@Na8 cubein-cube supramolecular building blocks has been synthesized, in which the unprecedented cubic (39.46)8 topological Mn8(μ4-OMe)6 cube with capped oxygen of five coordination is first revealed. Interestingly, the cube-in-cube Mn8@Na8 in combination with coordinated oxygen atoms makes up the unique nested Mn8O6@ O24@Na8 triple-shell structure, in which the inner Mn8 cube is penetrated by the O6 octahedron but is enclosed by a larger O24 truncated cube that is further surrounded by a still larger Na8 cube.
P
E)12 (Ln = lanthanides; E = S, Se) cores which can be viewed as a combination of the former two types.21 Encapsulation of a closed-shell heavy μ8-ion such as S, Se, Cl, and Br at the center of the Ln8 cube is possible.8 It was said that the heavy bridged, capped, or centered elements such P, S, Cl, Ge, As, and Te are indispensible in the stabilization of metal cubes.22 In spite of many metal organic cubes, only three metal-coordination frameworks using metal−organic cubes as SBBs have been reported to date.23−25 In this communication, we present a novel heterometalcoordination framework Na4Mn8(μ4-OCH3)6(μ4CH3CO2)12·2CH3CO2 1 based on unprecedented Mn8@Na8 cube-in-cube SBBs. The Mn8(μ4-O)6 cube in this compound is first revealed, which not only extends the metal atom of an M8 cube to the manganese element but also demonstrates that the lighter oxygen atom of alkoxide can act as the capped μ4element in M8(μ4-E)6 cubes. Solvothermal treatment of a mixture of Mn(CH3CO2)2, NaOH, CH3CN, and CH3OH at 160 °C generated light yellow cubic single crystals of 1 in a 65% yield (Figure 1). Infrared spectra, elemental analysis, and energy-disperse X-ray spectroscopy confirmed the formula of 1. The phase purity is confirmed by powered XRD. Compound 1 is thermally stable up to 240 °C. X-ray single crystallography revealed that compound 1 crystallizes in the cubic high symmetric space group Im3m ̅ ,
olyhedra were initially recognized by Plato and Archimedes thousands of years ago, which have been chemically replicated via inorganic, organic, and metal−organic methods.1−5 Recently, extended coordination network-based polyhedra as supramolecular building blocks (SBBs) are of current interest because such polyhedral SBBs typically start at the nanometer scale and possess an inherently aesthetic beauty and high symmetry.6 The simplest and most important polyhedra are the five Platonic polyhedra constructed from one type of regular polygon, meeting at identical vertices, and fifteen Archimedean polyhedra, consisting of single type of vertex and at least two different faces. Clearly, the evolution from the discrete polyhedra to polyhedral coordination frameworks would encompass different types of assemblies with various degrees of hierarchy. It should be noted that coordination frameworks in which polyhedra of different scales form nested polyhedron-in-polyhedron configurations are very rare.7−9 Platonic metal-coordination cubes are of importance in metal cluster, metal−metal bonding, and magnetochemistry.10−12 The first discovered metal-coordination cube belongs to the type of M8(μ2-E)12 (M = Cu, Ag; E = S, Se) cores, in which 12 edges of the cubic M8 unit are bridged by 12 μ2-E atoms in an array of an icosahedron.10,13 This type of metal cubes has been extended to magnetically important M8(CN)12 and heterometal M4M′4(CN)12 cores, in which cyanides in place of μ2-E act as bridged ligands to efficiently transfer magnetic interaction.14 The typical type of metal-coordination cube has M8(μ4-E)6 and metal-centered M9(μ4-E)6 cores (M = Fe, Co, Ni, Pd; E = S, P, As, Ge, Te), which can be viewed with the result of the interpenetration of a (μ4-E)6 octahedron by an M8 cube.13,15−20 The third type of metal-coordination cubes has Ln8(μ4-E)6(μ© 2013 American Chemical Society
Received: January 27, 2013 Revised: March 6, 2013 Published: March 11, 2013 1386
dx.doi.org/10.1021/cg400165v | Cryst. Growth Des. 2013, 13, 1386−1389
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OCH3)6(MeCO2)12 unit. The nearest and next nearest MnO6 within the Mn8(μ4-OCH3)6 cube are linked by edge- and corner-sharing with Mn···Mn distances being 3.1229(15) and 4.4146(10) Å. The Mn−O−Mn angles for edge-shared and corner-shared MnO 6 octahedra are 84.767(24)° and 144.845(24)°, respectively. It should be noted that six oxygen atoms of methanoxides also encompass a perfect octahedron. As mentioned before, the Mn8(μ4-O)6 core of the cube can be viewed by the result of the interpenetration of a (μ4-O)6 octahedron by a Mn8 cube, in which each Mn is linked to three nearest neighbors via edge-sharing and three next nearest neighbors via corner-sharing (Figure S3 of the Supporting Information). Polynuclear Mn clusters have been reviewed and topologically classified recently by Powell.26 Although a larger number of Mn clusters with different nuclearity and topologies have been reported, octanuclear Mn8(μ4-OCH3)6 cube has not been documented to date. In accordance with Powell’s classification, after considering the magnetic superexchange via the μ4-OMe groups, the Mn8(μ4-OCH3)6 cube has a 6connected topology and the total vertex symbol is (39.46)8 (Figure S3 of the Supporting Information). The remarkable feature of the Mn8(μ4-OCH3)6 metal organic cube is the lighter oxygen being the capped μ4-element, which has never been observed. The observation of the Mn8(μ4OCH3)6 cube demonstrates that besides the heaver elements such P, S, Cl, Ge, As, and Te, the light element oxygen also can act as a capped element to stabilize the M8(μ4-E)6 metal cubes. Compared with heavier main group elements such as P, S, Cl, Ge, As, and Te, oxygen without the d orbital generally has a coordination number not larger than four.27 Therefore, the fivecoordinated square pyramidal oxygen (four Mn and one C of Me) is very uncommon but not unprecedented. Another unique feature is that this is the first time to extend the metal in the M8(μ4-E)6 cubes to the Mn element. Besides the Platonic O6 octahedron and Mn8 cube, the Archimedean O24 truncated cube is enclosed if 24 carboxylate oxygen atoms are linked in the Mn8(μ4-OMe)6(MeCO2)12 unit (Figure 1). Within a truncated cube, all oxygen atoms are crystallographically equivalent, and there are two types of O···O distances, in agreement with the Archimedean-truncated cube. Finally, the O24 truncated cube is further encompassed by a still larger Na8 cube to finish the nested Mn8(μ4-O)6@O24@Na8 SBB (Figure 1f). Each nested Mn8(μ4-O)6@O24@Na8 SBB is linked to eight neighbors via sharing vertexes to form a threedimensional (3D) cationic structure of 1 with 3D channels (Figure 3). Actually, the channels are filled by free acetates to balance the charge. In accordance with the SBU theory proposed by Yaghi,28 the Mn8(μ4-OMe)6(MeCO2)12 clusters can be considered as 8connected nodes, which are linked by Na ions to form an uninodal eight-connected bcu network with the Schläfli symbol (424·64). Although the bcu net is a textbook example of one of five regular nets, coordination networks with the bcu net only appeared in a few examples where tetranuclear or pentanuclear clusters act as nodes.29 No bcu topological coordination network with cubic octanuclear clusters as nodes has been exemplified. The temperature-dependent magnetic susceptibilities of 1 were measured on crystalline samples in the temperature range of 2−300 K, under an applied field of 1000 Oe. The χmT value at room temperature is 11.32 cm3 K mol−1 per Mn2 unit (Figure 4), which is slightly lower than the spin-only value of 11.83 cm3 K mol−1 expected for two noninteracting high-spin
Figure 1. SEM of 1 showing cubic morphology.
and the asymmetric unit consists of one Mn(II) atom, one Na (I) atom, one methanoxide, and one acetate (Figure S2 in the Supporting Information). All atoms localize at special positions. The Mn(1) atom has symmetry of 3m and shows a distorted octahedral geometry, being coordinated by six O atoms separately from three methanoxides and three acetate groups. The Na(1) atom has a symmetry of −3m and also adopts an octahedral geometry, coordinated by six O atoms from six acetate groups. The Mn−O bond lengths are 2.248(3) and 2.3177(18) Å, being normal to the octahedral high spin Mn(II) atom. Na−O bond length of 2.341(3) Å is also normal to the NaO6 octahedra. The methanoxide in μ4-mode is capped to four Mn(II) atoms, while the acetate in the μ4-mode is bridged to two Mn(II) and two Na atoms. The charge balance requires that there are two free acetates per formula, which could not be located from the Fourier map due to the high symmetry of the structure. The eight Mn(II) atoms are arranged into a perfect cube that are capped by six methanoxides at six faces to form a Mn8(μ4OCH3)6 metal organic cube (Figure 2). The twelve edges of the Mn8(μ4-OCH3)6 cubes are occupied by twelve acetates via coordination to Mn atoms in syn-syn mode, forming a Mn8(μ4-
Figure 2. View of the (a) Mn8(μ4-OCH3)6 cube, (b) Na8(CH3CO2)12 cube, and (c) Na8Mn8(μ4-OCH3)6(CH3CO2)12 unit. Penetration of the Mn8 cube by the (d) O6 octahedron, (e) O24 truncated cube, and (f) nested Mn8O6@O24@Na8 SBB. 1387
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ASSOCIATED CONTENT
S Supporting Information *
Additional figures and IR, TG, XRPD, EDS, magnetization curve, and crystallographic cif data for 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the 973 Program (2012CB821701), the Ministry of Education of China (Grant IRT1156) and the National Science Fund for Distinguished Young Scholars (Grant 20925101).
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Figure 3. 3D cationic structure of 1 with 3D channels constructed by nested Mn8(μ4-O)6@O24@Na8 SBBs via the sharing of vertexes.
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Figure 4. Temperature dependence of χmT per Mn2 unit measured under an applied field of 1000 Oe for 1.
Mn(II) ions (S = 5/2, g = 2.0). Upon cooling, the χmT value monotonically decreases to a value of 0.83 cm3 K mol−1 at 2 K, characteristic of antiferromagnetic coupling. The isothermal field-dependent magnetization of 1 at 2 K steadily increases with H and is 2.20 Nβ per Mn2 unit at 50 kOe, in agreement with antiferromagnetic coupling. No magnetic hysteresis is observed. Generally, the edge-shared octahedra with the M− O−M angles close orthogonality of the magnetic orbital, while corner-shared octahedra tend to couple antiferromagnetically due to large M−O−M angles. Observation of overall antiferromagnetic behavior of 1 indicates that the antiferromagnetic interaction via a single oxygen bridge is stronger than the ferromagnetic interaction via double oxygen bridges, in alignment with Mn−O−Mn angles. In summary, a novel body-centered cubic heterometalcoordination network based on cube-in-cube SBBs has been synthesized and characterized. The cubic (39.46)8 topological Mn8(μ4-OMe)6 core with light capped oxygen atoms of five coordination is first revealed. The unique nested Mn8O6@ O24@Na8 SBB consists of the Mn8 cube and penetrated O6 octahedron, both of which are enclosed by a large O24 truncated cube that is surrounded by a still larger Na8 cube. Currently, we are trying to oxidize 1 into magnetically more important mixed-valent Mn8 cube cores and to make the DFT calculation to reveal the factors in the stabilization of usual square pyramidal five-coordinate oxygen. 1388
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